1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
The sequence of exposures, or shots, forms a printed pattern. When considering maskless lithography, each shot results from an image of a spatial light modulator (SLM) array being projected onto a photosensitive surface, such as a layer of photoresist on a wafer substrate. This results in deposition of a dose, or a quantity of irradiation from a light source, within a certain exposure zone on this surface. Exposure zones are created when the substrate surface is illuminated by flashes of light from the light source.
In a pixel grid imaging lithographic system utilizing a micro lens array (MLA), the MLA is one of the most critical optical components in the system. The MLA focuses patterned beam projected by the projection lenses so as to form a large array of image spots in the image plane, which coincides with the photoresist layer on the substrate surface. For optimal resolution of the projected image the image spots should be very accurately located, for example within about 20 nm relative to the idea grid in the XY plane and within a few microns (e.g., +/−3 microns) in the focal direction. This small tolerance can lead to errors. These errors can be randomly distributed and can be magnification errors in the X direction or in the Y direction or in both the X and Y directions, or can be positional errors of the order of about 0.1 to 1 micron.
However, it is extremely difficult to manufacture MLA's that are capable of generating image spots having position errors of less than about 20 nm, and currently there are no techniques available to permit correction of position errors in the XY plane to the required level of accuracy.
Therefore, what is needed is a system and method that at least partially compensate for an effect of manufacturing errors in a MLA when used for focusing a patterned beam projected onto a target portion of a substrate.